[ClangImporter] Do not import function decl if already imported via type signature (#85480)

This is a small incremental patch that is designed to address the issue
discovered by 9aa3c5d, which is that we
are importing various Decls multiple times.

rdar://164616956

Author: j-hui

lib/ClangImporter/ImportDecl.cpp

@@ -3853,8 +3853,8 @@ namespace {
       if (!dc)
         return nullptr;
 
-      // We may have already imported this function decl before we imported the
-      // parent record. In such a case it's important we don't re-import.
+      // We may have already imported this function decl while importing its
+      // decl context. Check decl cache to make sure we don't import twice.
       auto known = Impl.ImportedDecls.find({decl, getVersion()});
       if (known != Impl.ImportedDecls.end()) {
         return known->second;
@@ -3994,6 +3994,13 @@ namespace {
         return nullptr;
       }
 
+      // We may have already imported this function decl while importing its
+      // type signature. Check decl cache to make sure we don't import twice.
+      auto known2 = Impl.ImportedDecls.find({decl, getVersion()});
+      if (known2 != Impl.ImportedDecls.end()) {
+        return known2->second;
+      }
+
       if (name && name.isSimpleName()) {
         assert(importedName.hasCustomName() &&
                "imported function with simple name?");